"NYSE and Nasdaq are iconic . . . they are fierce competitors," said Christine Varney, assistant attorney general of the antitrust division of the Justice Department.
Shares of NYSE Euronext dropped $5.16, or 12.62 percent, to $35.73. Shares of Nasdaq OMX Group Inc. fell 68 cents to $26.23, while Atlanta-based IntercontinentalExchange Inc. (ICE) gained $3.90, or 3.30 percent, to $122.22.
Varney said all of the top 500 U.S. companies are listed on either the NYSE or Nasdaq and a merger would have substantially eliminated competition.
Though Nasdaq and NYSE are the two largest U.S. stock markets, both have been under pressure to find a merger partner because technology has driven down the cost of trading to almost nothing. -- AP